 When you think of air pollution, cars and factories, not small insects, probably come to mind. But according to Energy and Environment News, a recent study found that bark beetles have bored through more than 6 billion trees in the western U.S. and Canada and are causing the trees to release up to 20 times as much smog-causing compounds as healthy trees. As the bugs tunnel into the bark, they create pathways for the volatile organic compounds to be released.